ARM: dts: nomadik: Augment NHK15 panel setting

The NHK15 panel is specified inside the display controller,
which works for the DPI-type DT parsing the old fbdev code
used, but for the DRM driver it needs to be spawn as its own
device, so we move it out of the display controller.

We also drop the panel timings: this should be determined
by the hardware or a device-specific compatible string, not
by this type of encoding into the device tree. It turns out
that this hardware is strapped to the right configuration
at boot already and we the driver just reads out the
hardware-specified resolution and timings. Drop the
"panel,dpi" compatible string altogether.

Fix a comment error in the DTS file while we're at it.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>